1. Field of the Invention
This invention relates to a method for manufacturing easily and with a high productivity an aluminum or aluminum alloy clad steel sheet having the same degree of corrosion resistance as that of aluminum or an aluminum alloy, the same degree of strength as steel sheet and also the same degree of mechanical properties and formability as those of steel sheet of deep drawing quality and yet has the excellent adhesion.
2. Description of the Prior Art
Aluminized steel sheet which is prepared by applying aluminum or an aluminum alloy (hereinafter called "aluminum") to a steel sheet by hot dipping in general for improved corrosion resistance is in practical use. However, in view of the formability such as bendability, deep drawing quality, etc., the aluminum coating of the aluminized steel sheet is compelled to be thin, 40.mu. and less. It also has defects such as pin holes on the surface. Therefore, it has been hardly possible to obtain a sufficient degree of corrosion resistance of the aluminized steel sheet due to the defects such as pin holes on the surface.
On the other hand, an aluminum-steel, composite sheet which is prepared by combining the surface of steel with aluminum has been known to be to impart corrosion resistant and equally has sufficient strength. The composite sheet has conventionally been manufactured by an explosive bonding method or by a cladding method.
In the explosive pressure bonding method, an explosion force is utilized to achieve pressure bonding. This method results in a high manufacturing cost and also is unfit for mass production.
The cladding method is classified into a method of carrying out a cold rolling process after a cladding process and hot rolling process with heating after the cladding.
In the case of cold rolling, in order to obtain a good bonding effect, the draft reduction percentage of the rolling process must be set at least at 70%. This necessitates use of a powerful rolling mill and therefore needs large scale facilities. Further, since the degree of work done on the material is great, work hardening tends to take place and this makes it difficult to use a thin aluminum sheet as surface material and a thin steel sheet as core material.
In the other method wherein hot rolling is carried out after cladding, a draft reduction percentage of 15 to 55% suffices for obtaining a good bonding effect. However, with this method, it is difficult to control the cladding ratio because aluminum tends to elongate faster than steel. Furthermore, in order to obtain a perfect bond, the heat treatment must be carried out in a reductive atmosphere to prevent the aluminum and the steel from high temperature oxidation. This results in higher cost of facilities and processing operations than normal hot rolling.